For the measurement of particle emissions, various measuring devices have been developed, including for example various impactors and optical counters. The measuring devices are used to detect the particles to be analyzed and to produce information to draw up conclusions on the properties of the detected particles. In conventional impactors, in which particles with different aerodynamic dimensions are collected on different collecting substrates, information is obtained on the size distribution of the particles to be analyzed by weighing the mass of the particles collected on different collection substrates.
In more sophisticated impactors, such as an electrical low-pressure impactor, real-time electrical information is obtained on the size distribution of the particles, indicating variations in the particle distribution in real time. The electrical low-pressure impactor is described in more detail for example in the scientific article by Keskinen, Pietarinen and Lehtimäki, “Electrical low pressure impactor” published in the “Journal of Aerosol Science” [J. Aerosol Sci. Vol. 23, No. 4, pp. 353–360, 1992]. A copy of said article has been filed as an appendix to this application.
In addition to actual measuring devices, different particle classifiers are also known, including for example DMA devices (Differential Mobility Analyzer). The classifiers can be used to select from the particle flow under analysis a given subclass which is then led to the actual measuring device.
The article by W. P. Kelly and P. H. McMurry, “Measurement of Particle Density by Inertial Classification of Differential Mobility Analyzer-Generated Monodisperse Aerosol” [Aerosol Science and Technology 17:199–212, 1992], presents a method of prior art for measuring properties of a particle distribution by means of a DMA device and an impactor. FIG. 1 shows the principle of operation of this method.
In the method presented in the article, a flow 13a carrying the particle distribution to be analyzed is led to an apparatus 10 consisting of a DMA device 11 and an impactor 12. The flow is first led to the DMA device 11 which, by means of an electrical field, separates the particles with a narrow electrical range of mobility from the flow to a flow 13b to be led to the impactor 12. Particles whose electrical mobility is not within this narrow range are guided with flows 13c and 13d away from the measuring device 10.
By means of the DMA device, it has thus been possible to separate a monodispersive particle flow 13b with a given narrow electrical mobility distribution 14b from a polydispersive particle flow 13a with an electrical mobility distribution 14a led to the measuring device 10.
This monodispersive aerosol flow is then guided to the impactor 12 which subjects them to a classification based on the aerodynamic diameter. On the basis of this, it is possible to determine the aerodynamic size distribution 15 of particles contained in the flow 13b input in the impactor. When the adjustments of the DMA device 11 are known, it is possible to find out the median mobility of the monodispersive mobility distribution 14a included in the flow 13b that has passed through it.
Said article discloses how the average density of the monodispersive particle flow 13b can be determined by combining the aerodynamic size distribution 15 measured by the impactor 12 with the information on the median electrical mobility of the monodispersive particle flow 13b guided into the impactor.
The above-presented solution of prior art involves the problem that the density can only be determined for a narrow electrical mobility range at a time. In other words, by means of the method, the density can be computed for the monodispersive flow 13b selected by means of the DMA device 11. To determine the properties of a polydispersive flow 13a, this must be implemented, according to the above-presented solution of prior art, by scanning, i.e. by determining the density first in one electrical mobility range and then changing the adjustments of the DMA device in such a way that the measurement is made in another electrical mobility range. This procedure is repeated until the density has been determined in the whole range desired.
For the above-presented scanning measurement to produce reliable results, the flow 13a to be analyzed should remain unchanged during the whole measurement operation. Under real measuring conditions, there may be temporal variations in the flow to be analyzed, for which reason the above-presented solution of prior art is poorly suitable for the real-time measurement of a flow containing polydispersive particles under real conditions.
Another solution of prior art is disclosed in the article by Lehtimäki and Keskinen, “A method of modifying the sensitivity function of an aerosol photometer” published in the scientific journal “American Industrial Hygiene Association Journal” [Am. Ind. Hyg. Assoc. J 49 (8), 396–400 (1988)]. Also this article has been filed with the application. The article presents how a virtual impactor is placed in front of an optical particle counter, to separate particles with a large and a small diameter to different flows from the particle flow under analysis. From the separated flows, the flow containing large diameter particles is guided to the optical counter for the actual measurement.
Also this prior art solution involves the problem that the actual measurement operation does not relate to the actual flow to be analyzed but the flow to be analyzed has been modified before the measurement operation. Thus, it is not possible to obtain an overall conception of the properties of the particle distribution in the complete flow.